1. Field of the Invention
The invention is directed to a vibration damper with a frequency-selective damping force including an axially movable piston rod, a housing including an auxiliary piston and a main piston.
2. Description of the Related Art
A vibration damper with amplitude-dependent damping force is known from DE 10 2008 002 062 B3. In the embodiment of FIG. 1, a main piston is connected by a piston rod portion to an auxiliary piston inside a housing which is in turn fixed to a piston rod. With a connection channel inside a wall, the auxiliary piston forms a throttle valve. When the auxiliary piston is situated between the two connection channels, the main piston can be moved relative to the piston rod by a comparatively slight force because the damping medium located in the housing can be displaced from the housing via the connection channels. The main piston only generates a greater damping force as the auxiliary piston increasingly closes a connection channel.
In FIG. 2, the auxiliary piston additionally has damping valves which generate a damping force when the auxiliary piston closes a connection channel and is further displaced in direction of the end of the housing. Also, in this variant the main piston generates the substantial proportion of damping force. The auxiliary piston does not generate any damping force at all in a defined range and first assumes a kind of hydraulic pressure impact function at the end of the stroke. Therefore, this vibration damper also has an amplitude-selective damping force function.
In vibration dampers with a highly degressive damping force characteristic, piston rod vibrations which are perceived as knocking noises can occur during high frequency excitation of the vibration damper. In this regard, DE 10 2009 016 453 A1 suggests tuning a piston rod bearing support to a determined damping force. Unfortunately, this construction principle only functions at a single frequency but is not satisfactory over a greater frequency range.
It is thus an object of the present invention to provide a vibration damper in which unwanted piston rod vibrations are suppressed also at high frequency excitations.